Affiliation: Department of Obstetrics and Gynecology, Baylor College of Medicine, Houston, Texas, United States of America.

ABSTRACT

Background: Focal Dermal Hypoplasia (FDH) is a genetic disorder characterized by developmental defects in skin, skeleton and ectodermal appendages. FDH is caused by dominant loss-of-function mutations in X-linked PORCN. PORCN orthologues in Drosophila and mice encode endoplasmic reticulum proteins required for secretion and function of Wnt proteins. Wnt proteins play important roles in embryo development, tissue homeostasis and stem cell maintenance. Since features of FDH overlap with those seen in mouse Wnt pathway mutants, FDH likely results from defective Wnt signaling but molecular mechanisms by which inactivation of PORCN affects Wnt signaling and manifestations of FDH remain to be elucidated.

Conclusions: These data indicate that Porcn inactivation in the mouse produces a model for human FDH and that phenotypic features result from defective WNT signaling in ectodermal- and mesenchymal-derived structures.

Mentions:
We performed two sets of injections into blastocysts of mouse ES cells (line CSD256) that were targeted with a genetrap cassette containing a splice-acceptor sequence upstream of a β-galactosidase-neomycin fusion (β-geo) reporter gene inserted 3′ to exon 2, which contains the translation initiation site of Porcn (Figure 1A). Consistent with the prediction that this allele would result in inactivation of Porcn, only a single liveborn male chimera (Figure 1B–I) was found in 28 offspring. This animal had fused and hypoplastic digits on all 4 extremities (Figure 1B and 1E), 2 midline ventral wall defects, and a lower dorsal skin defect with alopecia (Figure 1C). The skin lesions showed a thin epidermis with presence of a dermoid cyst in one of the ventral lesions (Figure 1F). Interestingly, this mouse also had external male genitalia, but was a hermaphrodite with a single ovary with oviduct and a hypoplastic testis (Figure 1G). It developed a cystic intra-abdominal structure, suspected to be a dilated obstructed vas deferens (Figure 1D), and had a hydronephrotic kidney (Figure 1H). We confirmed the presence of the targeted allele in skin, liver, and tail of this mouse by PCR amplification of lacZ (Figure 1I). The phenotype in this mosaic male chimeric animal is consistent with the clinical presentation of FDH in males, who have somatic mosaic mutations [8], [9], [28]. The low survival of male chimeras is also in agreement with recent data showing that injection of CSD256 ES cells into tetraploid blastocysts results in early embryonic lethality and extensive gastrulation defects [27], [29]. This data provided the rationale for generation of a conditionally targeted Porcn allele to study in vivo effects of mammalian Porcn inactivation.

Mentions:
We performed two sets of injections into blastocysts of mouse ES cells (line CSD256) that were targeted with a genetrap cassette containing a splice-acceptor sequence upstream of a β-galactosidase-neomycin fusion (β-geo) reporter gene inserted 3′ to exon 2, which contains the translation initiation site of Porcn (Figure 1A). Consistent with the prediction that this allele would result in inactivation of Porcn, only a single liveborn male chimera (Figure 1B–I) was found in 28 offspring. This animal had fused and hypoplastic digits on all 4 extremities (Figure 1B and 1E), 2 midline ventral wall defects, and a lower dorsal skin defect with alopecia (Figure 1C). The skin lesions showed a thin epidermis with presence of a dermoid cyst in one of the ventral lesions (Figure 1F). Interestingly, this mouse also had external male genitalia, but was a hermaphrodite with a single ovary with oviduct and a hypoplastic testis (Figure 1G). It developed a cystic intra-abdominal structure, suspected to be a dilated obstructed vas deferens (Figure 1D), and had a hydronephrotic kidney (Figure 1H). We confirmed the presence of the targeted allele in skin, liver, and tail of this mouse by PCR amplification of lacZ (Figure 1I). The phenotype in this mosaic male chimeric animal is consistent with the clinical presentation of FDH in males, who have somatic mosaic mutations [8], [9], [28]. The low survival of male chimeras is also in agreement with recent data showing that injection of CSD256 ES cells into tetraploid blastocysts results in early embryonic lethality and extensive gastrulation defects [27], [29]. This data provided the rationale for generation of a conditionally targeted Porcn allele to study in vivo effects of mammalian Porcn inactivation.

Affiliation:
Department of Obstetrics and Gynecology, Baylor College of Medicine, Houston, Texas, United States of America.

ABSTRACT

Background: Focal Dermal Hypoplasia (FDH) is a genetic disorder characterized by developmental defects in skin, skeleton and ectodermal appendages. FDH is caused by dominant loss-of-function mutations in X-linked PORCN. PORCN orthologues in Drosophila and mice encode endoplasmic reticulum proteins required for secretion and function of Wnt proteins. Wnt proteins play important roles in embryo development, tissue homeostasis and stem cell maintenance. Since features of FDH overlap with those seen in mouse Wnt pathway mutants, FDH likely results from defective Wnt signaling but molecular mechanisms by which inactivation of PORCN affects Wnt signaling and manifestations of FDH remain to be elucidated.

Conclusions: These data indicate that Porcn inactivation in the mouse produces a model for human FDH and that phenotypic features result from defective WNT signaling in ectodermal- and mesenchymal-derived structures.